This project represents the integration of two former projects from the defunct Section on Comparative Studies in the Laboratory of Clinical Science (The Neurobiology of Attachment, Z01 MH 00797-LCS and Animal Models of Anxiety Z01 MH 02219-LCS). The central focus of this new project is the neural basis of affiliative behaviors; specifically the role of the neuropeptide oxytocin in several aspects of affiliation. Behavioral studies in the rat, which originally focused on maternal behavior, have been expanded to explore oxytocin's role in sexual behavior, social interaction, and the infant's response to separation. Cellular studies of the mechanisms of oxytocin's effects have continued to focus on the regulation of oxytocin receptor expression. These receptors in brain appear to be highly plastic. In the rat, expression of oxytocin receptors in specific brain regions is almost entirely dependent on gonadal steroids. By contrast, in selected regions of the mouse brain, oxytocin receptors may be inhibited by gonadal steroid stimulation. The oxytocin receptor has also emerged as an important factor in comparative studies of microtine rodents (voles) with contrasting patterns of affiliation. Last year we reported that the regional distribution of oxytocin receptors differed markedly between monogamous and polygamous voles. This year, we have explored the functional significance of these species differences in receptor distribution. In the monogamous prairie vole, oxytocin administered intracerebroventricularly induces a partner preference in the female and an increase in aggression in the male, effects which are critical to the initiation of a pair bond. Paternal behavior in this species releases oxytocin and is selectively decreased by amygdala lesions. In the polygamous montane vole, oxytocin has little influence on several measures of social behavior. The accumulated behavioral, receptor, and neuroanatomic evidence suggests that central oxytocin projections may have evolved in mammals to mediate diverse forms of attachment behavior.